Cytochrome c-mediated caspase-9 activation triggers apoptosis in Streptococcus pyogenes-infected epithelial cells

Epithelial cells are the initial sites of host invasion by group A Streptoc occus pyogenes (GAS), and their infection of epithelial cells has been sugg ested to induce apoptosis. However, the mechanism responsible for bacteria- host interaction and the induction of apoptosis has not been clearly unders tood. We demonstrate here that human pharyngeal epithelial HEp-2 cells beca me apoptotic with DNA fragmentation by invasion of GAS strains JRS4 (M6(+), F1(+)) and JRS145 (M6(-), F1(+) mutant of JRS4), whereas apoptotic cellula r changes were not observed in SAM1 (M6(+), F1(-) mutant) or SAM2 (M6(-), F 1(-) mutant) infected HEp-2 cells. Confocal microscopy revealed that Bax tr anslocation to mitochondria and cytochrome c release occurred after 4 h of infection. Western blot analyses showed that the amounts of Bcl-2 and Bcl-x (L) were decreased in the mitochondria of infected cells. In addition, we d emonstrated that the release of nuclear histone from infected cells was pre vented by the addition of caspase-9 inhibitor (Ac-LEHD-CHO). We conclude th at the internalization of GAS in epithelial cells is necessary and sufficie nt for the induction of apoptosis, which is initiated by mitochondrial dysf unction, and the mechanism of GAS-induced apoptosis is clearly different fr om that induced by other intracellular invasive bacteria, e.g. Shigella and Salmonella species.